Making a sketch of the triangles will help you see the problem. The diagram below shows the first five stacks of blocks:

You can answer the question by sketching the sixth, seventh, eighth, and ninth triangular stacks of blocks.

Another way to answer the question is to find a pattern. Make a table like the one in Example 5 on page 36:

Notice that the pattern is different from the one in the example, since the number of blocks does not increase by the same number. The number of blocks increases by the triangle number. That is, the pattern is dependent on the triangle number.